Autism specialists have long thought the disorder has a strong genetic component -– maybe stronger than any other neurodevelopmental condition.

Now a new study — the largest so far to look at autism in younger brothers and sisters of an affected child — finds their risk of also having autism is higher than previously thought, especially if the younger sibling is a boy.

The overall risk that a younger sibling of an autistic child will have the disorder is 19 percent, the researchers found. Earlier, smaller studies had pegged the risk at 3 to 14 percent.

But if the younger sibling is male, the risk shoots up to more than one in four, according to the study, which appears online in the journal Pediatrics. "That's a really high rate compared to the risk of autism in the general population, which is 1 in 110," says Sally Ozonoff of the University of California, Davis, who led the research.

By contrast, if the younger sibling is a girl, her risk of autism is 9 percent.

And if a family has two or more children with autism, the risk among younger siblings goes up even more — to 1 in 3.

The study involved 664 infants in the United States and Canada with at least one older sibling affected by autism, ranging from mild types called "pervasive developmental delay" and Asperger's syndrome to the most severe forms. These younger siblings were monitored from the age of six to eight months until 3 years to see if they showed signs of having a disorder on the autism spectrum.

The new numbers have implications for researchers, for families affected by autism, and for pediatricians everywhere who see the younger sibs of autistic children during their early well-child visits.

"It's the first thing families ask: How likely is this to happen again?" Ozonoff says. "Parents are already concerned, already watching their child's development carefully. We are able to supply some answers."

But Ozonoff says maybe the most important message is for pediatricians.

"If they have a little boy in their practice who has an older sibling [with autism], that child's risk is 25 times higher than another infant in their practice," she told Shots.

"This should mean there is more careful monitoring and screening beyond the usual questions at a normal well-child visit," Ozonoff continues. "Drilling down into the things that we know are early signs of autism — interest in people, responding to their name, responding to other people, smiling at other people."

Alycia Halladay of the advocacy group Autism Speaks, which partially funded the study along with the National Institutes of Health, agrees that the new data refute a passive wait-and-see attitude toward children who have an older sibling on the autism spectrum — disorders ranging from Asperger's to being profoundly uncommunicative.

"It is our hope," Halladay says, "that by encouraging parents to seek careful monitoring of their child, we can really push the whole health care system into doing a better job at providing an earlier diagnosis for those who are affected, so more kids who have autism are identified earlier."

That can really make a life-changing difference, says Judith Ursitti, a Massachusetts mother of two children with autism spectrum disorders. Her daughter, Amy, who is 11, has Asperger's, while her son, Jack, 7, has severe autism.

"For decades, many people have heard the word 'autism' and they just thought 'hopeless,' " Ursitti says. "But there are therapies that, if done early and intensively by the right sort of professionals, can really cause someone with autism to dramatically improve and overcome the challenges of autism."

She regrets that her son Jack didn't get diagnosed until after age 2, when he failed to speak or point, smile at others or wave goodbye — despite the fact that he had excellent pediatric care.

Enrolled in a special school for autistic children, supplemented by behavioral therapy at home, Jack Ursitti is now communicating "more than we ever thought he would," his mother says.

Judith Ursitti, who works with Autism Speaks as a lobbyist for better insurance coverage of autism diagnosis and treatment, says many parents of autistic children won't be surprised by the new study. In fact, they might have expected that the chance of a sibling being affected was much higher.

"It's amazing how families are affected more than once," Ursitti says. "I remember when Amy started getting intervention for Asperger's, and she was with a group of girls for therapy. And they all had brothers who had autism. And I thought, 'Wow, this is incredible!' "

But Ozonoff, the study leader, says the data might be reassuring for families who mistakenly believed they had a much higher risk of having a second or third child with autism. For every younger sibling who is affected, four others will not be.

"No matter how alarming these statistics are — and truly they are for researchers and clinicians and parents — I also tell people that even in the highest-risk groups ... the likelihood of their baby not having autism is always much higher," Ozonoff says. "So, holding that baby in their arms, there is reason to hope."

Transcript

ROBERT SIEGEL, host: Researchers have some important new information for families who have a child with autism. They now have a better idea of the risk of autism for that child's younger siblings. And as NPR's Richard Knox reports, it is higher than experts had thought.

RICHARD KNOX: It's one of the first questions parents ask after a child's been diagnosed with autism. What's the chance this could happen again? Earlier, smaller studies suggested it was between 3 and 14 percent, but the new research shows it's at least 19 percent. And Sally Ozonoff, who led the study, says for some children, the risk is even higher.

Dr. SALLY OZONOFF: If a family has a child with autism and now, for example, has a newborn boy, that baby's risk of having autism are one in four, essentially. And that's a really high rate compared to the general population, one in 110.

KNOX: Ozonoff says if there are two older siblings with autism, the risk jumps to one in three. She's a researcher at the University of California, Davis. But she says there are two ways of looking at this.

OZONOFF: No matter how alarming these statistics are - and truly, they are for us as researchers and clinicians and parents - I also tell families that even in the highest risk groups in this study, the likelihood of that new baby not having autism is always higher than the likelihood of them having autism.

KNOX: The new study appears in the journal Pediatrics. It has important implications for families wondering if a younger sibling of an autistic child may be affected, too.

Alycia Halladay works for Autism Speaks, an advocacy group that helped fund the new study.

Dr. ALYCIA HALLADAY: It tells parents who may show a concern, don't just wait and see. Take your child in consistently, and monitor them carefully. And it also is a message to health-care providers that they should be listening to the concerns of parents.

JUDITH URSITTI: OK, what do you want me to do?

JACK: Just spin me.

URSITTI: Spin you one more time?

JACK: Yeah.

URSITTI: OK. Ready?

JACK: Yeah, go.

URSITTI: Go.

KNOX: Judith Ursitti is twirling her son, Jack, in a swing at their home in suburban Boston. Jack, who's 7, is severely autistic. His 11-year-old sister, Amy, was diagnosed two years ago with Asperger's syndrome, a milder form of autism.

URSITTI: I missed those red flags, and my children weren't diagnosed as early as they could have been. So I do live in the world of shoulda-coulda-woulda, because they both had signs much earlier than they were diagnosed.

KNOX: Autism specialists say there's good evidence now that early diagnosis and treatment can offset many of the symptoms of autism: lack of interest in other people, reluctance to speak, learning problems, difficulty in making wants and needs known.

Ursitti worries that Jack's missed some of those chances.

URSITTI: And I don't know for sure but I really - I think it could have made a huge difference in his life had we known.

KNOX: Still, she says, the attention he's been getting the past three years is making a difference.

URSITTI: Between school and then treatment outside of school, medical treatment, he really started to learn to utter syllables, learned to nod his head yes or no, and then it just developed from there, you know. Now he can say, I want to put on my shoes - you know - or, I want to go swing.

KNOX: Researcher Sally Ozonoff says there's another important implication in the latest research. It has to do with the contribution of genes versus environment in autism. The new study clearly shows genes are important. But another study, published just last month, shows that there's a higher risk of fraternal twins both getting autism - roughly 30 percent. Ozonoff points out that genetically, fraternal twins are just like any two siblings who are not twins.

OZONOFF: That suggests that the shared intrauterine environment may also contribute to autism.

KNOX: So researchers are looking for things that pregnant women are exposed to that may help unravel the puzzle of just what causes autism.